1. Field of the Invention
The present invention relates to an image sensor including, as a light source, an EL light-emitting element which emits light by organic electroluminescence, and an image forming apparatus in which this image sensor is mounted. More particular, the invention relates to a structure for preventing a local temperature rise of an elongated EL light-emitting element.
2. Description of the Related Art
There are known multi-functional image forming apparatuses that are generally called “multi-function peripherals (MFP)” which have functions such as a scanner function, a printing function, a copy function and a network connection function.
This type of image forming apparatus includes a line-scan type image sensor which optically reads information, such as characters and images, from a to-be-scanned object such as a document. The image sensor includes a light source which radiates light on the to-be-scanned object. Conventionally, LED light-emitting elements have widely been used as such light sources.
On the other hand, in recent years, as disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2003-87502, for example, there have been attempts to replace general LED light-emitting elements in image sensors with EL light-emitting elements. The EL light-emitting element includes a line-shaped glass base plate which extends in a scanning direction and a light emission section which is formed on the surface of the glass base plate. The light emission section emits light by electroluminescence when an electric field is applied, and the light emission section is electrically connected to electrodes which are formed at end portions of the glass base plate.
The EL light-emitting element, together with a circuit board which drives the EL light-emitting element, is supported in a case which is formed of a synthetic resin. The circuit board has a plurality of circuit components including an LSI which produces heat in operation. When the EL light-emitting element is caused to emit light, the EL light-emitting element itself produces heat, and heat of the LSI conducts from the circuit board to the EL light-emitting element via the housing. Then, the EL light-emitting element is not uniformly heated since it has an elongated shape extending in the scanning direction. In particular, a part of the EL light-emitting element, which is near the LSI, is locally heated up to high temperatures. As a result, the temperature distribution in the longitudinal direction of the EL light-emitting element becomes non-uniform, and a temperature difference occurs in the EL light-emitting element.
If a temperature difference occurs in the EL light-emitting element, the distribution of electric current varies and a great amount of current tends to flow in a high-temperature part. Thus, the luminance of the high-temperature part of the EL light-emitting element increases, and conversely the luminance of a low-temperature part decreases. Consequently, non-uniformity in luminance distribution occurs in the longitudinal direction of the EL light-emitting element.
Hence, in the image sensor which includes the EL light-emitting element as the light source, when light is radiated on the to-be-scanned object, the illuminance on the illuminated surface of the to-be-scanned object becomes non-uniform in the scanning direction. Image information on the to-be-scanned object is led to a light-receiving element, such as a CCD, as optical density information. Thus, if a difference in illuminance occurs on the illuminated surface of the to-be-scanned object, there arises such a problem that the output of the image sensor deteriorates and the image quality on a part with low illuminance becomes inferior to the image quality on a part with high illuminance.
Jpn. Pat. Appln. KOKAI Publication No. 2006-197012 discloses an image sensor wherein heat radiation properties of a CCD are enhanced and the quality of an image is improved. According to this document, the CCD, which produces heat, is thermally connected to the housing of the image scanning apparatus via a heat conductive body. The heat conductive body transfers the heat that is produced by the CCD to the housing. Thereby, the heat of the CCD is radiated from the surface of the housing to the atmospheric air.
The invention described in the above document, however, is an invention for mainly enhancing heat radiation properties of the CCD. This invention is not intended to prevent occurrence of a temperature difference in the CCD when the CCD is affected by thermal influence from outside.
In other words, in the invention of the above document, even though the heat of the CCD can be let to escape from the heat conductive body to the housing, there is a concern that the CCD may have a part with a high temperature and a part with a low temperature, depending on the positional relationship between the CCD and the heat conductive body. Consequently, even if the heat radiation technique disclosed in the above document is applied to an elongated line-shaped EL light-emitting element, a temperature difference will inevitably occur in the EL light-emitting element. Therefore, this technique cannot be an effective solution to eliminate the non-uniformity in luminance distribution of the EL light-emitting element.
On the other hand, Jpn. Pat. Appln. KOKAI Publication No. H6-225077 discloses a technique wherein the light intensity of a light source is detected by using a reception-light sensor which is provided in a line-scan-type image sensor, and an image signal is corrected on the basis of the detected light intensity.
In the image sensor disclosed in the above document, however, a complex signal processing circuit is required in order to correct the light intensity that is detected by the reception-light sensor. Hence, even if a good image signal is obtained by correcting the light intensity, an increase in cost of the signal processing circuit is inevitable. This leads to such a problem that the price of the image sensor rises.